1. Field of the Invention
The present invention relates to a filler metal to be used for welding a part of porous sintered material to a part of the same material or steel in the assembling of automotive transmission systems.
2. Description of the Prior Art
The joining of steel to steel is accomplished mostly by arc welding or high energy density beam welding (such as electron beam welding and laser beam welding) These welding techniques, however, cannot be applied to sintered materials, because sintered materials contain pores, which, during welding, form large blowholes in the weld metal, resulting in insufficient weld strength There have been proposed several methods for joining sintered materials but they suffer from their respective disadvantages as follows.
(1) Joining by brazing
This involves difficulties in selecting brazing filler metals and establishing adequate brazing conditions and also presents problems associated with the wettability of brazing filler metal. With excess wettability, the brazing filler metal is absorbed by pores of base materials, resulting in a shortage of brazing filler metal on the joint interface. With poor wettability, the brazing filler metal does not achieve good joining of sintered materials, particularly those of medium and high carbon contents. In addition, brazing tends to grow pores near the joint interface, which is detrimental to uniform strength.
(2) Joining by shrinkage fit or expansion fit
This is accomplished during or after sintering by utilizing the difference of two parts in their coefficient of linear expansion. A joint by this method tends to lose its strength with time in some environment.
(3) Joining by infiltration
This joining method is by melting an infiltration material (such as copper) placed between two parts, thereby filling it into pores in the joint surface. Since the bond strength depends on the infiltration material, the joint strength is much lower than that of the base material. Fundamentally, this method cannot be applied to the joining of a part of steel to a part of sintered material.
(4) Joining by high energy density beam welding after infiltration
This joining method is disclosed in Japanese Patent Laid-open No. 160185/1990. According to this method, the beam welding is performed after the filling of pores with an infiltration material, which prevents the occurrence of blowholes. However, the beam welding with rapid cooling brings about transformation cracking and cold cracking when applied to sintered materials incorporated with 0.4-1% carbon for reinforcement. In addition, the infiltration material, which is usually copper, increases the copper content in the weld metal. This copper in conjunction with sulfur originating from the base metal makes the weld metal highly susceptible to hot cracking. Consisting of two steps of infiltration and beam welding, this method is poor in productivity.
(5) Joining by high energy density beam welding which employs a high-manganese filler material
This joining method was proposed by the present inventors. It is by high energy density beam welding which employs a high-manganese filler material incorporated with aluminum and titanium. It is free of all the shortcomings involved in the joining of sintered materials. The only one problem is that the weld metal is hard and poor in toughness because it is composed of Fe, C, and Mn.
As mentioned above, the conventional joining method and filler material are liable to give rise to several defects and poor in productivity and they do not provide sufficient joint strength and toughness. At present, there are no joining methods and filler materials which meet all the requirements.
If the above-mentioned high energy density beam welding can be applied to the joining of a part of porous sintered material to a part of the same material or steel, it would be advantageous in penetration depth, productivity, and strain. Unfortunately, defects such as cold cracking due to carbon and blowholes due to pores are inevitable in the joining of sintered materials, because sintered materials are porous and usually contain a considerable amount of carbon. (Small hot cracks are permissible, but large ones should be avoided.) For the beam welding to be of practical use in the field where high productivity is important, it is necessary that it does not give rise to cold cracking and blowholes but invariably provides joints of adequate strength.